1. Field of Invention
This invention relates to sensitizers for photopolymerizable compositions. More particularly, this invention relates to achieving high speed addition polymerization of ethylenically unsaturated monomers and oligomers by the use of merocyanine sensitizers in combination with photoinitiators such as diaryl iodonium salts, triaryl sulfonium salts, and trichloromethyl-s-triazines.
2. Background of the Art
Compounds capable of generating free radicals upon exposure to light, which initiate polymerization, are well known in the art ("Photopolymerization of Surface Coatings", C. G. Roffey, Wiley-Interscience Publications, 1982, pages 67-136). However, the utility of many of these photoinitiators or photoinitiator systems are quite limited because the initiators are activatible primarily in the ultraviolet region of the spectrum. For example, sulfonium and iodonium salt photoinitiators are known to absorb ultraviolet light only at wavelengths below about 300 nm, and trichloromethyl-s-triazines absorbs light below 350 nm.
Various investigations have been carried out in order to broaden the spectral response of photoinitiators. To achieve this, researchers have added compounds which absorb incident light more efficiently and then activate the photoinitiator to produce free radicals. These compounds which absorb the incident light more efficiently are generally termed as sensitizers or photosensitizers.
U.S. Pat. No. 3,729,313 describes sensitizers for diaryliodonium compounds for free radical polymerization. Sensitizers are included from the class of diphenylmethane, xanthane, acridine, methine and polymethine, thiazole, thiazine, azine, aminoketone, porphyrin, colored aromatic polycyclic hydrocarbons, p-substituted aminostyryl compounds and aminotriaryl methanes.
Japanese patent J6 0,088,005 (Agency of Ind. Sci. Tech.) discloses 3-ketocoumarin compounds as sensitizers for diaryliodonium compounds.
U.S. Pat. No. 4,250,053 describes sensitizers for Iodonium salts and particularly 1,3-diaryl-1-pyrazolines for free radical polymerization.
U.S. Pat. No. 3,617,288 describes sensitizers for halogenated hydrocarbon compounds.
U.S. Pat. No. 4,505,793 describes ketocoumarins as sensitizers for halogenated triazines for free radical polymerization.
While the aforementioned compositions have provided improved photospeed compared to the unsensitized system, there is still a need for a more sensitive photopolymer composition. High photospeed is particularly desirable for projection exposure imaging and imaging by laser scanning techniques.
U.S. Pat. Nos. 4,162,162, 4,268,667, 4,351,893 and European Patent 127,762 disclose sensitizers containing constrained amino-ketone groups for bi-imidazole initiators in addition polymerization. U.S. Pat. No. 4,505,793 discloses constrained coumarin sensitizers for triazine initiators. Amino group containing coumarins and constrained coumarins are also disclosed in U.S. Pat. Nos. 4,278,751, 4,147,552, 4,366,228 and G.B. Patent 2,083,832 for use as triplet sensitizers for cyclo-addition reactions or for use as sensitizers for radical polymerization in combination with arylaminoacetic acids. References of these compounds are also found in Polym. Eng. Sci. 1983, 23, 1022-1024. This article mentions the utility of aminoketocoumarin compounds as sensitizers with alkoxypyridinium salts. Photoreactions of coumarin compounds are also mentioned in J. Org. Chem. 1984, 49, 2705-2708. U.S. Pat. No. 4,250,053 teaches constrained coumarin as a sensitizer of iodonium salt for cationic polymerization.
Although constrained alkylamino ketone sensitizers are known, none of the prior art teaches its use to increase the rate of polymerization when used in combination with iodonium salts or halogenated triazines (however, see below), compared with the non-constrained alkylaminoketone sensitizers. Moreover, increase of photospeed by the incorporation of constrained group (particularly julolidine group) in aminoketone (merocyanine) sensitizers in combination with iodonium salts or halogenated triazines is surprising. For example, U.S. Pat. No. 4,250,053 (3M), examples 18, 20 and 21 teaches that julolidine containing sensitizers (sensitizers 77 and 79), showed slower curing rate than non-constrained sensitizer (sensitizer 72) for epoxy curing.
In U.S. Pat. No. 4,505,793 Fuji describes ketocoumarin sensitizers with halogenated compounds (including triazine) for free radical polymerization. Although no example is given showing the speed enhancement imparted by julolidine containing sensitizers, nevertheless, the following sensitizer is disclosed as one of the preferred sensitizers. ##STR1##
Our measurements on this sensitizer have not shown the speed enhancement found with the sensitizers of the present invention, and we have found that for ketocoumarin sensitizers, the best sensitization is achieved when the julolidine group is on the ketocoumarin ring as shown below: ##STR2##